(a) Field of the Invention
The present invention relates generally to soft decision decoding of digital data. More particularly, it relates to a system and method that uses soft decision decoding to provide a high receiver gain across low bandwidth communication links.
(b) Description of Related Art
Hard decision decoding methods for digital data transmissions are well known in the art. Hard decision decoding methods detect and/or correct bit errors by resolving the received bits into either a logical xe2x80x9c1xe2x80x9d or a xe2x80x9c0.xe2x80x9d Forward error correction (FEC) block codes and convolutional codes are examples of the hard decision decoding methods that are in widespread use today.
Soft decision decoding methods are also known in the art. Soft decision decoding quantizes a received bit signal into more than two states or levels. For example, a five-bit soft decision decoding method quantizes received bit signals into thirty-two possible levels depending on their xe2x80x9cclosenessxe2x80x9d to either a logical xe2x80x9c1xe2x80x9d or xe2x80x9c0.xe2x80x9d These additional levels provide a measure of certainty or confidence that is associated with the bit values. Soft decision decoding methods use this confidence information to perform a variety of known statistical operations and algorithms that work to increase receiver gain, particularly in fading channels. These soft decoding schemes typically provide more receiver gain than would be possible with hard decoding schemes alone.
Soft decision decoding schemes are especially beneficial in receivers, such as cellular base stations, that receive wireless transmissions from one or more mobile user units because channel fading is a common and significant problem in these systems. Conventional soft decision decoding systems and methods, however, do not provide a workable solution to the fading problem for these existing cellular systems. Conventional soft decision systems and methods can provide higher receiver gain only by increasing the number of soft decision bits used to represent each and every bit signal. Thus, with conventional systems, system throughput may be substantially compromised by higher gains because each and every bit signal must carry the soft decision bit overhead across the communication link. Furthermore, system throughput requirements (i.e. the number of users) often limit these soft decision decoding schemes to three bits, which provides inadequate receiver gain in fading channels. Thus, there is a need for a system and method of soft decision decoding that improves receiver gain without compromising system throughput.
Accordingly, the present invention provides a method of receiving and processing a digital bitstream that provides increased receiver gain without compromising system throughput. The present invention is embodied in a soft decision encoding/decoding scheme, wherein a bitstream may be transmitted across a low bandwidth communication link as temporally spaced bursts of messages bits within a frame. A headend of the communication link individually processes and packages the bursts with a scale factor for transmission across the link. The scale factor may vary semi-dynamically (i.e., on a burst by burst basis). This scale factor is used at the tailend of the communication link to rescale, map, or expand the soft decision bits into a soft decision scheme having more bits than the scheme used at the headend of the communication link. Thus, using the present invention, the semi-dynamic gain information is sent once with each burst and does not produce bit overhead for each and every bit signal of the burst.
In accordance with the present invention, the average magnitude of the bit signals within a burst is measured and used to associate a scale factor with the burst. If the average magnitude of the bit signals is low then a scale factor providing a high gain is associated with the burst. Conversely, if the average magnitude of the bit signals is high then a scale factor providing lower gain is associated with the burst. The individual bit signals of the burst are represented using a multi-bit soft decision scheme that generates soft decision values based on the scale factor associated with the burst and the normalized bit signal value (i.e. the bit signal""s closeness to a logic xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d). The scale factor and the soft decision values for the burst are then packaged together and transmitted across the communication link.
At the tailend of the communication link, the scale factor is stripped off the packaged burst of soft decision values/bits. The soft decision bits are then rescaled or mapped (i.e., expanded) into a multi-bit scheme having more soft decision bits than the scheme used at the headend of the communication link. This resealing or mapping is accomplished by combining the information contained in the scale factor for the burst with each of the soft decision values within the burst.
Accordingly, the present invention may be embodied in a soft decision decoding system and method that generates an average signal magnitude based on a plurality of message bits. A scale factor is generated in accordance with this average signal magnitude, and soft decision bits are generated by processing the message bits using the scale factor. Preferably, the soft decision bits are generated using a quantization having at least two bits, and soft decision bits are rescaled using at least five bits. The scale factor and soft decision bits are transmitted across a communication link, and received at a destination (e.g., a base station of a cellular communications system). The soft decision bits are resealed at the destination using the scale factor, then rounded to the nearest integer value. The rounded values are then decoded.
The present invention may also be embodied in an apparatus for receiving a plurality of message bits having a soft decision processor, a rescaler coupled to the soft decision processor, and a decoder coupled to the rescaler. The soft decision processor generates an average signal magnitude from the plurality of message bits, then generates a scale factor based on the average signal magnitude. The soft decision processor generates soft decision bits by quantizing the plurality of message bits in accordance with the scale factor. The rescaler generates rescaled soft decision bits using the scale factor and a quantization having at least four bits. The decoder generates decoded message bits from the rescaled soft decision bits using a soft decision algorithm.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.